Process fluid regulators are used throughout industry to control fluid flow within process systems. Generally, process fluid regulators include a regulator body having a fluid inlet and a fluid outlet connected by a fluid flow passage. A movable control element, such as a plug, cooperates with a seat to vary a size of an opening within the fluid flow passage, which controls the flow of process fluid through the process fluid regulator. Actuators are attached to the regulator body and connected to the control element to operatively control a position of the control element with respect to the seat. Actuators can include a housing and a diaphragm disposed within the housing, the diaphragm dividing the inside of the housing into at least to chambers. The diaphragm is connected to the control element by an actuator stem so that the control element moves in conjunction with the diaphragm. A control spring biases the diaphragm to a desired position, which biases the control element to a desired position (i.e., to an open position or a closed position). Process fluid pressure (either upstream of the control member or downstream of the control member) may be used as an input to the actuator to offset or overcome spring force generated by the control spring. The actuator uses the input process fluid pressure to adjust the position of the control element (by way of the diaphragm and stem) to maintain a desired downstream process fluid pressure. In this way, the process fluid regulator “regulates” the downstream pressure of the process fluid.
In some systems, the process fluid is a liquid that is flowing through the system at high pressure. When a high pressure process liquid is used as an input to the actuator of a process fluid regulator, residual gas may become trapped in one of the chambers, thereby reducing the available volume within the chamber for the process liquid. This is especially true when the process fluid regulator is installed in an upright orientation, with the actuator located above the regulator body. A reduction in available volume may reduce actuator effectiveness and thus, the effectiveness of the process fluid regulator itself.